In biology, a refugium (plural: refugia) is a location which supports an isolated or relictpopulation of a once more widespread species. This isolation (allopatry) can be due to climatic changes, geography, or human activities such as deforestation and overhunting.
Mountain gorilla
Present examples of refugial animal species are the mountain gorilla, isolated to specific mountains in central Africa, and the Australian sea lion,
isolated to specific breeding beaches along the south-west coast of
Australia, due to humans taking so many of their number as game. This
resulting isolation, in many cases, can be seen as only a temporary
state; however, some refugia may be longstanding, thereby having many endemic species, not found elsewhere, which survive as relict populations. The Indo-Pacific Warm Pool has been proposed to be a longstanding refugium, based on the discovery of the "living fossil" of a marine dinoflagellate called Dapsilidinium pastielsii, currently found in the Indo-Pacific Warm Pool only.
In anthropology, refugia often refers specifically to Last Glacial Maximum refugia, where some ancestral human populations may have been forced back to glacial refugia (similar small isolated pockets on the face of the continental ice sheets) during the last glacial period. Going from west to east, suggested examples include the Franco-Cantabrian region (in northern Iberia), the Italian and Balkan peninsulas, the Ukrainian LGM refuge, and the Bering Land Bridge.
Archaeological and genetic data suggest that the source populations of
Paleolithic humans survived the glacial maxima (including the Last Glacial Maximum) in sparsely wooded areas and dispersed through areas of high primary productivity while avoiding dense forest cover.
Glacial refugia, where human populations found refuge during the last
glacial period, may have played a crucial role in shaping the emergence
and diversification of the language families that exist in the world
today.
More recently, refugia has been used to refer to areas that could offer relative climate stability in the face of modern climate change.
Speciation
As an example of a locale refugia study, Jürgen Haffer first proposed the concept of refugia to explain the biological diversity of bird populations in the Amazonian river basin. Haffer suggested that climatic change in the late Pleistocene led to reduced reservoirs of habitable forests in which populations become allopatric. Over time, that led to speciation: populations of the same species that found themselves in different refugia evolved differently, creating parapatricsister-species. As the Pleistocene ended, the arid conditions gave way to the present humid rainforest environment, reconnecting the refugia.
Scholars have since expanded the idea of this mode of speciation
and used it to explain population patterns in other areas of the world,
such as Africa, Eurasia, and North America.
Theoretically, current biogeographical patterns can be used to infer
past refugia: if several unrelated species follow concurrent range patterns, the area may have been a refugium. Moreover, the current distribution of species with narrow ecological requirements tend to be associated with the spatial position of glacial refugia.
Simple environment examples of temperature
Two slopes with different sunlight exposure; only one is covered with snow
One can provide a simple explanation of refugia involving core temperatures and exposure to sunlight. In the northern hemisphere, north-facing sites on hills or mountains, and places at higher elevations count as cold sites. The reverse are sun- or heat-exposed, lower-elevation, south-facing sites: hot sites. (The opposite directions apply in the southern hemisphere.)
Each site becomes a refugium, one as a "cold-surviving refugium" and
the other as a "hot-surviving refugium". Canyons with deep hidden areas
(the opposite of hillsides, mountains, mesas, etc. or other exposed
areas) lead to these separate types of refugia.
A concept not often referenced is that of "sweepstakes colonization":
when a dramatic ecological event occurs, for example a meteor strike,
and global, multiyear effects occur. The sweepstake-winning species
happens to already be living in a fortunate site, and their environment
is rendered even more advantageous, as opposed to the "losing" species,
which immediately fails to reproduce.
Ecological
understanding and geographic identification of climate refugia that
remained significant strongholds for plant and animal survival during
the extremes of past cooling and warming episodes largely pertain to the
Quaternary glaciation cycles during the past several million years, especially in the Northern Hemisphere.
A number of defining characteristics of past refugia are prevalent,
including "an area where distinct genetic lineages have persisted
through a series of Tertiary or Quaternary climate fluctuations owing to
special, buffering environmental characteristics", "a geographical
region that a species inhabits during the period of a
glacial/interglacial cycle that represents the species' maximum
contraction in geographical range," and "areas where local populations
of a species can persist through periods of unfavorable regional
climate."
Future climate change refugia
In systematic conservation planning, the term refugium has been used to define areas that could be used in protected area development to protect species from climate change. The term has been used alternatively to refer to areas with stable habitats or stable climates. More specifically, the term in situ
refugium is used to refer to areas that will allow species that exist
in an area to remain there even as conditions change, whereas ex situ refugium refers to an area into which species distributions can move to in response to climate change. Sites that offer in situ refugia are also called resilient sites in which species will continue to have what they need to survive even as climate changes.
One study found with downscaled climate models that areas near the coast are predicted to experience overall less warming than areas toward the interior of the US State of Washington. Other research has found that old-growth forests are particularly insulated from climatic changes due to evaporative cooling effects from evapotranspiration and their ability to retain moisture. The same study found that such effects in the Pacific Northwest would create important refugia for bird species. A review of refugia-focused conservation strategy in the Klamath-Siskiyou Ecoregion
found that, in addition to old-growth forest, the northern aspects of
hillslopes and deep gorges would provide relatively cool areas for
wildlife and seeps or bogs surrounded by mature and old-growth forests would continue to supply moisture even as water availability decreases.
The Lake Pedder area had high geodiversity before it was flooded.
Beginning in 2010 the concept of geodiversity (a term used previously in efforts to preserve scientifically important geological features) entered into the literature of conservation biologists
as a potential way to identify climate change refugia and as a
surrogate (in other words, a proxy used when planning for protected
areas) for biodiversity. While the language to describe this mode of conservation planning
hadn't fully developed until recently, the use of geophysical diversity
in conservation planning goes back at least as far as the work by Hunter
and others in 1988, and Richard Cowling and his colleagues in South Africa also used "spatial features" as surrogates for ecological processes in establishing conservation areas in the late 1990s and early 2000s. The most recent efforts have used the idea of land facets (also referred to as geophysical settings, enduring features, or geophysical stages), which are unique combinations of topographical features (such as slope steepness, slope direction, and elevation) and soil composition, to quantify physical features. The density of these facets, in turn, is used as a measure of geodiversity. Because geodiversity has been shown to be correlated with biodiversity,
even as species move in response to climate change, protected areas
with high geodiversity may continue to protect biodiversity as niches get filled by the influx of species from neighboring areas.
Highly geodiverse protected areas may also allow for the movement of
species within the area from one land facet or elevation to another.
Conservation scientists, however, emphasize that the use of
refugia to plan for climate change is not a substitute for fine-scale
(more localized) and traditional approaches to conservation, as
individual species and ecosystems will need to be protected where they exist in the present.
They also emphasize that responding to climate change in conservation
is not a substitute for actually limiting the causes of climate change.
The thylacine (Thylacinus cynocephalus) is an example of a recently extinct species.
Palaeotherium is an example of an extinct genus that is only recorded from fossil records before the existence of hominids.
Extinction is the termination of a taxon by the death of its last member. A taxon may become functionally extinct before the death of its last member if it loses the capacity to reproduce and recover. Because a species' potential range
may be very large, determining this moment is difficult, and is usually
done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "reappears" (typically in the fossil record) after a period of apparent absence.
Through evolution, species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition. The relationship between animals and their ecological niches has been firmly established. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive with little to no morphological change for hundreds of millions of years.
Mass extinctions
are relatively rare events; however, isolated extinctions of species
and clades are quite common, and are a natural part of the evolutionary
process. Only recently have extinctions been recorded and scientists have become alarmed at the current high rate of extinctions.
Most species that become extinct are never scientifically documented.
Some scientists estimate that up to half of presently existing plant and
animal species may become extinct by 2100. A 2018 report indicated that the phylogenetic diversity of 300 mammalian species erased during the human era since the Late Pleistocene would require 5 to 7 million years to recover.
According to the 2019 Global Assessment Report on Biodiversity and Ecosystem Services by IPBES,
the biomass of wild mammals has fallen by 82%, natural ecosystems have
lost about half their area and a million species are at risk of
extinction—all largely as a result of human actions. Twenty-five percent
of plant and animal species are threatened with extinction.
In a subsequent report, IPBES listed unsustainable fishing, hunting and
logging as being some of the primary drivers of the global extinction
crisis.
In June 2019, one million species of plants and animals were at
risk of extinction. At least 571 plant species have been lost since
1750, but likely many more. The main cause of the extinctions is the
destruction of natural habitats by human activities, such as cutting
down forests and converting land into fields for farming.
A dagger symbol (†) placed next to the name of a species or other taxon normally indicates its status as extinct.
Examples
Examples of species and subspecies that are extinct include:
A species is extinct when the last existing member dies. Extinction
therefore becomes a certainty when there are no surviving individuals
that can reproduce and create a new generation. A species may become functionally extinct
when only a handful of individuals survive, which cannot reproduce due
to poor health, age, sparse distribution over a large range, a lack of
individuals of both sexes (in sexually reproducing species), or other reasons.
Pinpointing the extinction (or pseudoextinction) of a species requires a clear definition of that species.
If it is to be declared extinct, the species in question must be
uniquely distinguishable from any ancestor or daughter species, and from
any other closely related species. Extinction of a species (or
replacement by a daughter species) plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge.
Skeleton of various extinct dinosaurs; some other dinosaur lineages still flourish in the form of birds
In ecology, extinction is sometimes used informally to refer to local extinction,
in which a species ceases to exist in the chosen area of study, despite
still existing elsewhere. Local extinctions may be made good by the
reintroduction of individuals of that species taken from other
locations; wolf reintroduction is an example of this. Species that are not globally extinct are termed extant. Those species that are extant, yet are threatened with extinction, are referred to as threatened or endangered species.
Currently, an important aspect of extinction is human attempts to
preserve critically endangered species. These are reflected by the
creation of the conservation status"extinct in the wild" (EW). Species listed under this status by the International Union for Conservation of Nature (IUCN) are not known to have any living specimens in the wild and are maintained only in zoos
or other artificial environments. Some of these species are
functionally extinct, as they are no longer part of their natural
habitat and it is unlikely the species will ever be restored to the
wild. When possible, modern zoological institutions try to maintain a viable population for species preservation and possible future reintroduction to the wild, through use of carefully planned breeding programs.
The extinction of one species' wild population can have knock-on
effects, causing further extinctions. These are also called "chains of
extinction". This is especially common with extinction of keystone species.
A 2018 study indicated that the sixth mass extinction started in the Late Pleistocene could take up to 5 to 7 million years to restore mammal diversity to what it was before the human era.
Extinction of a parent species where daughter species or subspecies
are still extant is called pseudoextinction or phyletic extinction.
Effectively, the old taxon vanishes, transformed (anagenesis) into a successor, or split into more than one (cladogenesis).
Pseudoextinction is difficult to demonstrate unless one has a
strong chain of evidence linking a living species to members of a
pre-existing species. For example, it is sometimes claimed that the
extinct Hyracotherium, which was an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkey; however, as fossil species typically leave no genetic material behind, one cannot say whether Hyracotheriumevolved into more modern horse species
or merely evolved from a common ancestor with modern horses.
Pseudoextinction is much easier to demonstrate for larger taxonomic
groups.
A Lazarus taxon or Lazarus species refers to instances where a
species or taxon was thought to be extinct, but was later rediscovered.
It can also refer to instances where large gaps in the fossil record of a
taxon result in fossils reappearing much later, although the taxon may
have ultimately become extinct at a later point.
The coelacanth, a fish related to lungfish and tetrapods,
is an example of a Lazarus taxon that was known only from the fossil
record and was considered to have been extinct since the end of the Cretaceous Period. In 1938, however, a living specimen was found off the Chalumna River (now Tyolomnqa) on the east coast of South Africa. Calliostoma bullatum, a species of deepwater sea snail originally described from fossils in 1844 proved to be a Lazarus species when extant individuals were described in 2019.
Attenborough's long-beaked echidna (Zaglossus attenboroughi) is an example of a Lazarus species from Papua New Guinea that had last been sighted in 1962 and believed to be possibly extinct, until it was recorded again in November 2023.
Some species currently thought to be extinct have had continued
speculation that they may still exist, and in the event of rediscovery
would be considered Lazarus species. Examples include the thylacine, or Tasmanian tiger (Thylacinus cynocephalus), the last known example of which died in Hobart Zoo in Tasmania in 1936; the Japanese wolf (Canis lupus hodophilax), last sighted over 100 years ago; the American ivory-billed woodpecker (Campephilus principalis), with the last universally accepted sighting in 1944; and the slender-billed curlew (Numenius tenuirostris), not seen since 2007.
Causes
The passenger pigeon, one of the hundreds of species of extinct birds, was hunted to extinction over the course of a few decades.
As long as species have been evolving, species have been going
extinct. It is estimated that over 99.9% of all species that ever lived
are extinct. The average lifespan of a species is 1–10 million years,
although this varies widely between taxa.
A variety of causes can contribute directly or indirectly to the
extinction of a species or group of species. "Just as each species is
unique", write Beverly and Stephen C. Stearns, "so is each extinction ... the causes for each are varied—some subtle and complex, others obvious and simple". Most simply, any species that cannot survive and reproduce
in its environment and cannot move to a new environment where it can do
so, dies out and becomes extinct. Extinction of a species may come
suddenly when an otherwise healthy species is wiped out completely, as
when toxicpollution renders its entire habitat
unliveable; or may occur gradually over thousands or millions of years,
such as when a species gradually loses out in competition for food to
better adapted competitors. Extinction may occur a long time after the
events that set it in motion, a phenomenon known as extinction debt.
Assessing the relative importance of genetic factors compared to
environmental ones as the causes of extinction has been compared to the
debate on nature and nurture. The question of whether more extinctions in the fossil record have been caused by evolution or by competition or by predation or by disease or by catastrophe is a subject of discussion; Mark Newman, the author of Modeling Extinction, argues for a mathematical model that falls in all positions. By contrast, conservation biology uses the extinction vortex model to classify extinctions by cause. When concerns about human extinction have been raised, for example in Sir Martin Rees' 2003 book Our Final Hour, those concerns lie with the effects of climate change or technological disaster.
Human-driven extinction started as humans migrated out of Africa more than 60,000 years ago.
Currently, environmental groups and some governments are concerned with
the extinction of species caused by humanity, and they try to prevent
further extinctions through a variety of conservation programs. Humans can cause extinction of a species through overharvesting, pollution, habitat destruction, introduction of invasive species (such as new predators and food competitors), overhunting, and other influences. Explosive, unsustainable human population growth and increasing per capita consumption are essential drivers of the extinction crisis. According to the International Union for Conservation of Nature
(IUCN), 784 extinctions have been recorded since the year 1500, the
arbitrary date selected to define "recent" extinctions, up to the year
2004; with many more likely to have gone unnoticed. Several species have
also been listed as extinct since 2004.
If adaptation increasing population fitness is slower than environmental degradation plus the accumulation of slightly deleterious mutations, then a population will go extinct.
Smaller populations have fewer beneficial mutations entering the
population each generation, slowing adaptation. It is also easier for
slightly deleterious mutations to fix in small populations; the resulting positive feedback loop between small population size and low fitness can cause mutational meltdown.
Limited geographic range is the most important determinant of genus extinction at background rates but becomes increasingly irrelevant as mass extinction arises.
Limited geographic range is a cause both of small population size and
of greater vulnerability to local environmental catastrophes.
Extinction rates can be affected not just by population size, but by any factor that affects evolvability, including balancing selection, cryptic genetic variation, phenotypic plasticity, and robustness. A diverse or deep gene pool
gives a population a higher chance in the short term of surviving an
adverse change in conditions. Effects that cause or reward a loss in genetic diversity can increase the chances of extinction of a species. Population bottlenecks can dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and make inbreeding more frequent.
Extinction sometimes results for species evolved to specific ecologies that are subjected to genetic pollution—i.e., uncontrolled hybridization, introgression and genetic swamping that lead to homogenization or out-competition from the introduced (or hybrid) species. Endemic populations can face such extinctions when new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact. Extinction is likeliest for rare species coming into contact with more abundant ones; interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool (for example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the abundant domestic water buffalo). Such extinctions are not always apparent from morphological (non-genetic) observations. Some degree of gene flow
is a normal evolutionary process; nevertheless, hybridization (with or
without introgression) threatens rare species' existence.
The gene pool of a species or a population is the variety of genetic information in its living members. A large gene pool (extensive genetic diversity) is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) reduces the range of adaptions possible. Replacing native with alien genes narrows genetic diversity within the original population, thereby increasing the chance of extinction.
Habitat degradation is currently the main anthropogenic cause of
species extinctions. The main cause of habitat degradation worldwide is
agriculture, with urban sprawl, logging, mining, and some fishing practices close behind. The degradation of a species' habitat may alter the fitness landscape
to such an extent that the species is no longer able to survive and
becomes extinct. This may occur by direct effects, such as the
environment becoming toxic,
or indirectly, by limiting a species' ability to compete effectively
for diminished resources or against new competitor species.
Habitat destruction, particularly the removal of vegetation that
stabilizes soil, enhances erosion and diminishes nutrient availability
in terrestrial ecosystems. This degradation can lead to a reduction in
agricultural productivity. Furthermore, increased erosion contributes to
poorer water quality by elevating the levels of sediment and pollutants
in rivers and streams.
Habitat degradation through toxicity can kill off a species very rapidly, by killing all living members through contamination or sterilizing
them. It can also occur over longer periods at lower toxicity levels by
affecting life span, reproductive capacity, or competitiveness.
Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforests and replacement with open pastureland is widely cited as an example of this; elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example, a fern
that depends on dense shade for protection from direct sunlight can no
longer survive without forest to shelter it. Another example is the
destruction of ocean floors by bottom trawling.
Diminished resources or introduction of new competitor species also often accompany habitat degradation. Global warming
has allowed some species to expand their range, bringing competition to
other species that previously occupied that area. Sometimes these new
competitors are predators and directly affect prey species, while at
other times they may merely outcompete vulnerable species for limited
resources. Vital resources including water and food can also be limited during habitat degradation, leading to extinction.
In the natural course of events, species become extinct for a number
of reasons, including but not limited to: extinction of a necessary
host, prey or pollinator, interspecific competition,
inability to deal with evolving diseases and changing environmental
conditions (particularly sudden changes) which can act to introduce
novel predators, or to remove prey. Recently in geological time, humans
have become an additional cause of extinction of some species, either as
a new mega-predator or by transportinganimals and plants
from one part of the world to another. Such introductions have been
occurring for thousands of years, sometimes intentionally (e.g. livestock released by sailors on islands as a future source of food) and sometimes accidentally (e.g. rats escaping from boats). In most cases, the introductions are unsuccessful, but when an invasive alien species does become established, the consequences can be catastrophic. Invasive alien species can affect native species directly by eating them, competing with them, and introducing pathogens or parasites
that sicken or kill them; or indirectly by destroying or degrading
their habitat. Human populations may themselves act as invasive
predators. According to the "overkill hypothesis", the swift extinction
of the megafauna in areas such as Australia (40,000 years before present), North and South America (12,000 years before present), Madagascar, Hawaii
(AD 300–1000), and New Zealand (AD 1300–1500), resulted from the sudden
introduction of human beings to environments full of animals that had
never seen them before and were therefore completely unadapted to their
predation techniques.
Coextinction refers to the loss of a species due to the extinction of another; for example, the extinction of parasitic insects following the loss of their hosts. Coextinction can also occur when a species loses its pollinator, or to predators in a food chain
who lose their prey. "Species coextinction is a manifestation of one of
the interconnectednesses of organisms in complex ecosystems ... While
coextinction may not be the most important cause of species extinctions,
it is certainly an insidious one." Coextinction is especially common when a keystone species goes extinct. Models suggest that coextinction is the most common form of biodiversity loss. There may be a cascade of coextinction across the trophic levels. Such effects are most severe in mutualistic and parasitic relationships. An example of coextinction is the Haast's eagle and the moa:
the Haast's eagle was a predator that became extinct because its food
source became extinct. The moa were several species of flightless birds
that were a food source for the Haast's eagle.
Extinction as a result of climate change has been confirmed by fossil studies. Particularly, the extinction of amphibians during the Carboniferous Rainforest Collapse, 305 million years ago.
A 2003 review across 14 biodiversity research centers predicted that,
because of climate change, 15–37% of land species would be "committed to
extinction" by 2050. The ecologically rich areas that would potentially suffer the heaviest losses include the Cape Floristic Region and the Caribbean Basin.
These areas might see a doubling of present carbon dioxide levels and
rising temperatures that could eliminate 56,000 plant and 3,700 animal
species. Climate change has also been found to be a factor in habitat loss and desertification.
Sexual selection and male investment
Studies of fossils following species from the time they evolved to their extinction show that species with high sexual dimorphism,
especially characteristics in males that are used to compete for
mating, are at a higher risk of extinction and die out faster than less
sexually dimorphic species, the least sexually dimorphic species
surviving for millions of years while the most sexually dimorphic
species die out within mere thousands of years. Earlier studies based on
counting the number of currently living species in modern taxa have
shown a higher number of species in more sexually dimorphic taxa which
have been interpreted as higher survival in taxa with more sexual
selection, but such studies of modern species only measure indirect
effects of extinction and are subject to error sources such as dying and
doomed taxa speciating more due to splitting of habitat ranges into
more small isolated groups during the habitat retreat of taxa
approaching extinction. Possible causes of the higher extinction risk in
species with more sexual selection shown by the comprehensive fossil
studies that rule out such error sources include expensive sexually
selected ornaments having negative effects on the ability to survive natural selection, as well as sexual selection
removing a diversity of genes that under current ecological conditions
are neutral for natural selection but some of which may be important for
surviving climate change.
The blue graph shows the apparent percentage (not the absolute number) of marine animalgenera
becoming extinct during any given time interval. It does not represent
all marine species, just those that are readily fossilized. The labels
of the traditional "Big Five" extinction events and the more recently
recognised Capitanian mass extinction event are clickable links; see Extinction event for more details. (source and image info)
There have been at least five mass extinctions in the history of life
on earth, and four in the last 350 million years in which many species
have disappeared in a relatively short period of geological time. A
massive eruptive event that released large quantities of tephra particles into the atmosphere is considered to be one likely cause of the "Permian–Triassic extinction event" about 250 million years ago, which is estimated to have killed 90% of species then existing. There is also evidence to suggest that this event was preceded by another mass extinction, known as Olson's Extinction. The Cretaceous–Paleogene extinction event (K–Pg) occurred 66 million years ago, at the end of the Cretaceous period; it is best known for having wiped out non-avian dinosaurs, among many other species.
The changing distribution of the world's land mammals in tonnes of carbon. The biomass of wild land mammals has declined by 85% since the emergence of humans.
According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70% believed that the Earth is currently in the early stages of a human-caused mass extinction, known as the Holocene extinction.
In that survey, the same proportion of respondents agreed with the
prediction that up to 20% of all living populations could become extinct
within 30 years (by 2028). A 2014 special edition of Science declared there is widespread consensus on the issue of human-driven mass species extinctions. A 2020 study published in PNAS
stated that the contemporary extinction crisis "may be the most serious
environmental threat to the persistence of civilization, because it is
irreversible."
Biologist E. O. Wilson estimated
in 2002 that if current rates of human destruction of the biosphere
continue, one-half of all plant and animal species of life on earth will
be extinct in 100 years.
More significantly, the current rate of global species extinctions is
estimated as 100 to 1,000 times "background" rates (the average
extinction rates in the evolutionary time scale of planet Earth),faster than at any other time in human history, while future rates are likely 10,000 times higher. However, some groups are going extinct much faster. Biologists Paul R. Ehrlich and Stuart Pimm, among others, contend that human population growth and overconsumption are the main drivers of the modern extinction crisis.
In January 2020, the UN's Convention on Biological Diversity
drafted a plan to mitigate the contemporary extinction crisis by
establishing a deadline of 2030 to protect 30% of the Earth's land and
oceans and reduce pollution by 50%, with the goal of allowing for the
restoration of ecosystems by 2050. The 2020 United Nations' Global Biodiversity Outlook
report stated that of the 20 biodiversity goals laid out by the Aichi
Biodiversity Targets in 2010, only 6 were "partially achieved" by the
deadline of 2020.
The report warned that biodiversity will continue to decline if the
status quo is not changed, in particular the "currently unsustainable
patterns of production and consumption, population growth and
technological developments". In a 2021 report published in the journal Frontiers in Conservation Science,
some top scientists asserted that even if the Aichi Biodiversity
Targets set for 2020 had been achieved, it would not have resulted in a
significant mitigation of biodiversity loss. They added that failure of
the global community to reach these targets is hardly surprising given
that biodiversity loss is "nowhere close to the top of any country's
priorities, trailing far behind other concerns such as employment,
healthcare, economic growth, or currency stability."
History of scientific understanding
Tyrannosaurus, one of the many extinct dinosaur genera. The cause of the Cretaceous–Paleogene extinction event is a subject of much debate amongst researchers.Georges Cuvier's 1812 unpublished version of the skeletal reconstruction of Anoplotherium commune with muscles. Today, the Paleogene mammal is thought to have gone extinct from the Grande Coupure extinction event in western Europe.Georges Cuvier compared fossil mammoth jaws to those of living elephants, concluding that they were distinct from any known living species.
For much of history, the modern understanding of extinction as the end of a species
was incompatible with the prevailing worldview. Prior to the 19th
century, much of Western society adhered to the belief that the world
was created by God and as such was complete and perfect. This concept reached its heyday in the 1700s with the peak popularity of a theological concept called the great chain of being, in which all life on earth, from the tiniest microorganism to God, is linked in a continuous chain.
The extinction of a species was impossible under this model, as it
would create gaps or missing links in the chain and destroy the natural
order. Thomas Jefferson was a firm supporter of the great chain of being and an opponent of extinction, famously denying the extinction of the woolly mammoth on the grounds that nature never allows a race of animals to become extinct.
A series of fossils were discovered in the late 17th century that
appeared unlike any living species. As a result, the scientific
community embarked on a voyage of creative rationalization, seeking to
understand what had happened to these species within a framework that
did not account for total extinction. In October 1686, Robert Hooke presented an impression of a nautilus to the Royal Society that was more than two feet in diameter, and morphologically distinct from any known living species. Hooke theorized that this was simply because the species lived in the deep ocean and no one had discovered them yet. While he contended that it was possible a species could be "lost", he thought this highly unlikely. Similarly, in 1695, Sir Thomas Molyneux published an account of enormous antlers found in Ireland that did not belong to any extant taxa in that area. Molyneux reasoned that they came from the North American moose and that the animal had once been common on the British Isles.
Rather than suggest that this indicated the possibility of species
going extinct, he argued that although organisms could become locally
extinct, they could never be entirely lost and would continue to exist
in some unknown region of the globe. The antlers were later confirmed to be from the extinct deerMegaloceros.
Hooke and Molyneux's line of thinking was difficult to disprove. When
parts of the world had not been thoroughly examined and charted,
scientists could not rule out that animals found only in the fossil
record were not simply "hiding" in unexplored regions of the Earth.
Georges Cuvier is credited with establishing the modern conception of extinction in a 1796 lecture to the French Institute, though he would spend most of his career trying to convince the wider scientific community of his theory.
Cuvier was a well-regarded geologist, lauded for his ability to
reconstruct the anatomy of an unknown species from a few fragments of
bone. His primary evidence for extinction came from mammoth skulls found near Paris.
Cuvier recognized them as distinct from any known living species of
elephant, and argued that it was highly unlikely such an enormous animal
would go undiscovered. In 1798, he studied a fossil from the Paris Basin that was first observed by Robert de Lamanon
in 1782, first hypothesizing that it belonged to a canine but then
deciding that it instead belonged to an animal that was unlike living
ones. His study paved the way to his naming of the extinct mammal genus Palaeotherium in 1804 based on the skull and additional fossil material along with another extinct contemporary mammal genus Anoplotherium. In both genera, he noticed that their fossils shared some similarities with other mammals like ruminants and rhinoceroses but still had distinct differences. In 1812, Cuvier, along with Alexandre Brongniart and Geoffroy Saint-Hilaire, mapped the strata of the Paris basin.
They saw alternating saltwater and freshwater deposits, as well as
patterns of the appearance and disappearance of fossils throughout the
record.From these patterns, Cuvier inferred historic cycles of catastrophic
flooding, extinction, and repopulation of the earth with new species.
Cuvier's fossil evidence showed that very different life forms
existed in the past than those that exist today, a fact that was
accepted by most scientists. The primary debate focused on whether this turnover caused by extinction was gradual or abrupt in nature.
Cuvier understood extinction to be the result of cataclysmic events
that wipe out huge numbers of species, as opposed to the gradual decline
of a species over time. His catastrophic view of the nature of extinction garnered him many opponents in the newly emerging school of uniformitarianism.
Jean-Baptiste Lamarck, a gradualist and colleague of Cuvier, saw the fossils of different life forms as evidence of the mutable character of species.
While Lamarck did not deny the possibility of extinction, he believed
that it was exceptional and rare and that most of the change in species
over time was due to gradual change.
Unlike Cuvier, Lamarck was skeptical that catastrophic events of a
scale large enough to cause total extinction were possible. In his
geological history of the earth titled Hydrogeologie, Lamarck instead
argued that the surface of the earth was shaped by gradual erosion and
deposition by water, and that species changed over time in response to
the changing environment.
Charles Lyell, a noted geologist and founder of uniformitarianism,
believed that past processes should be understood using present day
processes. Like Lamarck, Lyell acknowledged that extinction could occur,
noting the total extinction of the dodo and the extirpation of indigenous horses to the British Isles. He similarly argued against mass extinctions, believing that any extinction must be a gradual process.
Lyell also showed that Cuvier's original interpretation of the Parisian
strata was incorrect. Instead of the catastrophic floods inferred by
Cuvier, Lyell demonstrated that patterns of saltwater and freshwater deposits, like those seen in the Paris basin, could be formed by a slow rise and fall of sea levels.
The concept of extinction was integral to Charles Darwin's On the Origin of Species, with less fit lineages disappearing over time. For Darwin, extinction was a constant side effect of competition. Because of the wide reach of On the Origin of Species, it was widely accepted that extinction occurred gradually and evenly (a concept now referred to as background extinction). It was not until 1982, when David Raup and Jack Sepkoski
published their seminal paper on mass extinctions, that Cuvier was
vindicated and catastrophic extinction was accepted as an important
mechanism.
The current understanding of extinction is a synthesis of the
cataclysmic extinction events proposed by Cuvier, and the background
extinction events proposed by Lyell and Darwin.
Human attitudes and interests
A great hammerhead caught by a sport fisherman. Human exploitation now threatens the survival of this species. Overfishing is the primary driver of shark population declines, which have fallen over 71% since 1970.
Extinction is an important research topic in the field of zoology, and biology in general, and has also become an area of concern outside the scientific community. A number of organizations, such as the Worldwide Fund for Nature, have been created with the goal of preserving species from extinction. Governments have attempted, through enacting laws, to avoid habitat destruction, agricultural over-harvesting, and pollution.
While many human-caused extinctions have been accidental, humans have
also engaged in the deliberate destruction of some species, such as
dangerous viruses,
and the total destruction of other problematic species has been
suggested. Other species were deliberately driven to extinction, or
nearly so, due to poaching or because they were "undesirable", or to push for other human agendas. One example was the near extinction of the American bison, which was nearly wiped out by mass hunts sanctioned by the United States government, to force the removal of Native Americans, many of whom relied on the bison for food.
Biologist Bruce Walsh states three reasons for scientific interest in the preservation of species: genetic resources, ecosystem stability, and ethics; and today the scientific community "stress[es] the importance" of maintaining biodiversity.
In modern times, commercial and industrial interests often have
to contend with the effects of production on plant and animal life.
However, some technologies with minimal, or no, proven harmful effects
on Homo sapiens can be devastating to wildlife (for example, DDT). BiogeographerJared Diamond notes that while big business
may label environmental concerns as "exaggerated", and often cause
"devastating damage", some corporations find it in their interest to
adopt good conservation practices, and even engage in preservation
efforts that surpass those taken by national parks.
Governments sometimes see the loss of native species as a loss to ecotourism, and can enact laws with severe punishment against the trade in native species in an effort to prevent extinction in the wild. Nature preserves are created by governments as a means to provide continuing habitats to species crowded by human expansion. The 1992 Convention on Biological Diversity has resulted in international Biodiversity Action Plan
programmes, which attempt to provide comprehensive guidelines for
government biodiversity conservation. Advocacy groups, such as The
Wildlands Project and the Alliance for Zero Extinctions, work to educate the public and pressure governments into action.
People who live close to nature can be dependent on the survival
of all the species in their environment, leaving them highly exposed to
extinction risks. However, people prioritize day-to-day survival over species conservation; with human overpopulation in tropical developing countries, there has been enormous pressure on forests due to subsistence agriculture, including slash-and-burn agricultural techniques that can reduce endangered species's habitats.
Antinatalist philosopher David Benatar
concludes that any popular concern about non-human species extinction
usually arises out of concern about how the loss of a species will
impact human wants and needs, that "we shall live in a world
impoverished by the loss of one aspect of faunal diversity, that we
shall no longer be able to behold or use that species of animal." He
notes that typical concerns about possible human extinction, such as the
loss of individual members, are not considered in regards to non-human
species extinction. Anthropologist Jason Hickel
speculates that the reason humanity seems largely indifferent to
anthropogenic mass species extinction is that we see ourselves as
separate from the natural world and the organisms within it. He says
that this is due in part to the logic of capitalism:
"that the world is not really alive, and it is certainly not our kin,
but rather just stuff to be extracted and discarded – and that includes
most of the human beings living here too."
Biologist Olivia Judson has advocated the deliberate extinction of certain disease-carrying mosquito species. In a September 25, 2003 article in The New York Times,
she advocated "specicide" of thirty mosquito species by introducing a
genetic element that can insert itself into another crucial gene, to
create recessive "knockout genes". She says that the Anopheles mosquitoes (which spread malaria) and Aedes mosquitoes (which spread dengue fever, yellow fever, elephantiasis,
and other diseases) represent only 30 of around 3,500 mosquito species;
eradicating these would save at least one million human lives per year,
at a cost of reducing the genetic diversity of the family Culicidae
by only 1%. She further argues that since species become extinct "all
the time" the disappearance of a few more will not destroy the ecosystem:
"We're not left with a wasteland every time a species vanishes.
Removing one species sometimes causes shifts in the populations of other
species—but different need not mean worse." In addition, anti-malarial
and mosquito control programs offer little realistic hope to the 300 million people in developing nations
who will be infected with acute illnesses this year. Although trials
are ongoing, she writes that if they fail "we should consider the
ultimate swatting."
Biologist E. O. Wilson has advocated the eradication of several species of mosquito, including malaria vector Anopheles gambiae.
Wilson stated, "I'm talking about a very small number of species that
have co-evolved with us and are preying on humans, so it would certainly
be acceptable to remove them. I believe it's just common sense."
There have been many campaigns – some successful – to locally eradicate tsetse flies and their trypanosomes in areas, countries, and islands of Africa (including Príncipe).
There are currently serious efforts to do away with them all across
Africa, and this is generally viewed as beneficial and morally
necessary, although not always.
Cloning
The Pyrenean ibex,
the only animal to have been brought back from extinction and the only
one to go extinct twice. The ibex apparently only lived for several
minutes.
Some, such as Harvard geneticist George M. Church, believe that ongoing technological advances will let us "bring back to life" an extinct species by cloning, using DNA from the remains of that species. Proposed targets for cloning include the mammoth, the thylacine, and the Pyrenean ibex.
For this to succeed, enough individuals would have to be cloned, from
the DNA of different individuals (in the case of sexually reproducing
organisms) to create a viable population. Though bioethical and philosophical objections have been raised, the cloning of extinct creatures seems theoretically possible.
In 2003, scientists tried to clone the extinct Pyrenean ibex (C. p. pyrenaica). This attempt failed: of the 285 embryos reconstructed, 54 were transferred to 12 Spanish ibexes and ibex–domestic goat hybrids, but only two survived the initial two months of gestation before they, too, died.
In 2009, a second attempt was made to clone the Pyrenean ibex: one
clone was born alive, but died seven minutes later, due to physical
defects in the lungs.
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